CMOS image sensors and CCD image sensors have been developed as solid-state image sensor devices that are incorporated in digital cameras, mobile-phone cameras, and the like. In the CMOS image sensor, MOSFET transistors are used for charge transfer from photoelectric conversion elements such as photodiodes (an insulated-gate field effect transistor is designated as MOSFET or MOS in the present specification). For reading out electric charge accumulated through photoelectric conversion, i.e., image information, it is required to perform independent-timing control for a multiplicity of MOSFET transistors included in the CMOS image sensor. Hence, it is common practice to form a plurality of layers for power supply lines and switching data lines in the CMOS image sensor (as disclosed in JP-A-2000-78475).
With an increase in the number of pixels, each pixel size has been reduced significantly, giving rise to the challenging design issue of how to enhance the efficiency of light collection. It is possible to enhance the efficiency of light collection by providing such an arrangement that incident light passing through a microlens is focused onto a photodiode. However, due to further integration density of pixels, there arises considerable difficulty in the optical design of the microlens. Where a focal length is increased, incident light is obstructed by a light-shielding film and/or a metal layer formed over the photodiode. Alternatively, where the focal length is decreased, incident light impinges on other parts than the photodiode. Thus, in the microlens optical design, it is difficult to achieve a satisfactory level of enhancement with respect to the efficiency of light collection.
As a solution to the above difficulty, a pixel structure using an optical waveguide has been devised. In this pixel structure, there is provided an optical waveguide for confinedly introducing incident light to a photodiode. When light is incident from a high-refractive-index material onto a low-refractive-index material, total reflection takes place depending on the angle of incidence thereof. Hence, in common practice of optical waveguide fabrication, a material having a refractive index higher than that of a dielectric layer surrounding an optical waveguide is formed on an interior wall of the optical waveguide or is embedded inside the optical waveguide. In fabrication of another type of optical waveguide, a cylindrical element having high-reflectivity metal coated on an interior wall thereof is employed.
In JP-A-2003-249632, there is disclosed an optical waveguide having a metallic film coated on an interior wall thereof. In this optical waveguide, an inlet thereof and a light-shielding film disposed as a topmost metallic layer are coupled to each other without gap, and a microlens is adjusted to have a focus position thereof in the vicinity of an open area of the light-shielding film so that light is securely incident into the optical waveguide and reflected by the metallic film coated on the interior wall to guidedly impinge on a photodiode.
In JP-A-2002-198508, there is disclosed a CCD type of solid-state image sensor device wherein transfer electrodes are configured in a single layer formed on the same plane via narrow gaps. The single layer is arranged with unnecessary space eliminated so as not to obstruct light incidence onto a photodiode, wiring lines thereof being disposed around the photodiode. A light-shielding film is made of nonconductive material, and a space available for wiring line arrangement is fully unitized to form a pixel part in a structure having merely one metal layer.